Hitherto, diverse attempts have been reported in developing drowsiness sensors intended for vehicle drivers, including (1) one adapted to detect the images of drivers' faces by cameras to determine their drowsiness by the motions of their eyelids, (2) one adapted to monitor heartbeats or pulses, and (3) one adapted to monitor changes in electrical resistance of the skin between wrists and little fingers. All of them, however, still have many disadvantages to be overcome and have not been put to practical use.
More specifically, the abovementioned methods have, for example, the following disadvantages: (1) the image monitoring method has a disadvantage in that a sudden change in the amount of light, such as during daytime and night, makes it difficult to recognize images, (2) the heartbeat or pulse monitoring method is disadvantageous in that it is difficult to distinguish between composure of experienced drivers and drowsiness, and (3) the method for monitoring changes in the electrical resistance of skin has no scientific basis and it is difficult to determine such changes, depending on humidity of skin.
Meanwhile, the present inventor has previously invented a drowsiness sensor (registration number 2009287) based on the fact that if eyelids close for 0.5 second or more, then it indicates heavy drowsiness, independently of differences among individuals.